Demlo Number

The initially palindromic numbers 1, 121, 12321, 1234321, 123454321, ... (Sloane's A002477). For the first through ninth terms, the sequence is given by the generating function

(1)

(Plouffe 1992, Sloane and Plouffe 1995).

The definition of this sequence is slightly ambiguous from the tenth term on, but the most common convention follows from the following observation. The sequences of consecutive and reverse digits  and , respectively, are given by

			(2)
			(3)

for , so the first few Demlo numbers are given by

			(4)
			(5)

But, amazingly, this is just the square of the th repunit , i.e.,

(6)

for , and the squares of the first few repunits are precisely the Demlo numbers: , , , ... (Sloane's A002275 and A002477). It is therefore natural to use (6) as the definition for Demlo numbers  with , giving 1, 121, ..., 12345678987654321, 1234567900987654321, 123456790120987654321, ....

The equality  for  also follows immediately from schoolbook multiplication, as illustrated above. This follows from the algebraic identity

(7)

The sums of digits of the Demlo numbers for  are given by

(8)

More generally, for , 2, ..., the sums of digits are 1, 4, 9, 16, 25, 36, 49, 64, 81, 82, 85, 90, 97, 106, ... (Sloane's A080151). The values of  for which these are square are 1, 2, 3, 4, 5, 6, 7, 8, 9, 36, 51, 66, 81, ... (Sloane's A080161), corresponding to the Demlo numbers 1, 121, 12321, 1234321, 123454321, 12345654321, 1234567654321, 123456787654321, 12345678987654321, 12345679012345679012345679012345678987654320987654320987654320987654321, ... (Sloane's A080162).

Ref: mathworld.wolfram.com